Monoclonal B-Cell Lymphocytosis: The Silent Clone the Haematologists Should Not Neglect

Monoclonal B-cell lymphocytosis (MBL) is a hematologic condition defined by the presence of a small clonal B-cell population in the peripheral blood, without clinical or laboratory evidence of lymphoproliferative disorders. It is classified into low-count (LC-MBL) and high-count (HC-MBL) subtypes based on clonal size. HC-MBL shares genetic and biological features with early-stage chronic lymphocytic leukemia (CLL), has a well-established 1%–2% annual risk of progression, and is associated with an increased risk of infections, second malignancies and a reduced response to vaccines due to immune dysfunction. LC-MBL, on the other hand, is a distinct entity with a lower likelihood of progression, but its potential impact on immune function remains unclear. While some studies suggest an association with increased infection risk and immune alterations, further research is needed to clarify its clinical significance. Beyond the risk of progression, MBL is increasingly recognized as a condition requiring careful management due to its broader implications on immune function and cancer susceptibility. Given these risks, preventive strategies are essential for all individuals with MBL, including adherence to cancer screening programs, vaccinations, smoking cessation, sun protection, and a healthy lifestyle to mitigate potential complications.